Abstract

The aim of the study was to determine whether the application of phosphate compounds (phosphorite
rock, phosphate fertilizer) to polyminerallic waste rocks can inhibit sulfide oxidation and metal mobility
(Cu, Pb, Zn, Cd, Ni, Mn, Mg). Waste rocks comprised sulfidic carbonaceous shales and were sourced from
the Century Pb–Zn mine, NW Queensland, Australia. The acid producing, Pb–Zn rich rocks consisted of
major quartz, muscovite/illite, dolomite, siderite and kaolinite as well as smaller amounts of sulfide minerals
(e.g. galena, sphalerite, pyrite). Laboratory leach experiments were conducted on finely granulated
phosphate-treated waste rocks (>2 to <30 mm) over 13 weeks, whereas phosphate amendment of coarsely
granulated waste rocks (sand to boulder size) was investigated using heap leach piles at the mine site
over an 11 months period. Results of the laboratory experiments demonstrate that the treatment of finely
granulated waste rocks with phosphorite rock produced leachates with near-neutral pH values due to
calcite dissolution. This in turn did not allow the leaching of apatite, formation of secondary phosphate
phases and phosphate stabilization to occur. Metal mobility in these amended wastes was restricted by
the dissolution of calcite and the resultant near-neutral pH conditions. By contrast, the application of the
water-soluble phosphate fertilizer MKP (KH2PO4) to polyminerallic sulfidic waste rocks during the shortterm
laboratory experiments led to the formation of phosphate coatings and precipitates and inhibited
acid and metal release (Cd, Mn, Ni, Pb, Zn). At least in the short term, the application of phosphate fertilizers
proved to be an effective method. However, results of the long-term field trials demonstrate that
coarsely granulated waste rocks were not coated by secondary phosphate phases and that amendment by
phosphorite rock or superphosphate fertilizer did not improve leachate quality compared to the unamended
waste. Thus, phosphate stabilization appears ineffective in suppressing oxidation of sulfides in
coarsely granulated mine wastes.